Conventional technologies in applying a photopolarimeter for measuring the polarization state of a light transmission is faced with the limitations that the manufacture of the photopolarimeters still require manual efforts to align and assemble multiple photodetectors according to predefined optical paths. Furthermore, these multiple photodetectors must maintain at fixed relative positions to achieve reliable and accurate measurements. These requirements may not be easily satisfied when a photopolarimeter is employed to operate under the conditions where the environment may impose humid, heat, vibrations, or other hostile conditions on the photopolarimeters.
In a U.S. Pat. No. 4,681,450 Azzam discloses a photopolarimeter for the simultaneous measurement of all four Stokes parameters of light. The light beam, the state of polarization of which is to be determined, strikes, at oblique angles of incidence, three photodetector surfaces in succession, each of which is partially specularly reflecting and each of which generates an electrical signal proportional to the fraction of the radiation it absorbs. A fourth photodetector is substantially totally light absorbtive and detects the remainder of the light. The four outputs thus developed form a 4×1 signal vector which is linearly related to the input Stokes vector S. Consequently, S is obtained by S=A−1 I. The 4×4 instrument matrix A is a nonsingular matrix that requires the planes of incidence for the first three detector surfaces are all different. For a given arrangement of four detectors, A can be either computed or determined by calibration. Thus Azzam disclose a configuration for arranging photodetectors to measure the state of polarization (SOP) of a light. However, since the actual implementation of the arrangement of the photodetectors as disclosed would involve manual operations to place these photodetectors at specific locations, practical implementation of the disclosed invention for manufacturing a device for measuring the SOP of a light would not be economically feasible due to a lack of manufacturability.
A configuration of four-detector photopolarimeter using a corner cube configuration was disclosed by Jian Liu and R. M. A. Azzam, “A corner-cube four-detector photopolarimeter,” Optics and Laser Technology 29(5), 233-238 (1997), and “Polarization properties of corner-cube retroreflectors: theory and experiment,” Applied Optics 36, 1553-1559 (1997). The disclosures made in these two published papers are hereby incorporated herein as reference of this Application. The disclosures made in these papers however presented further difficulties that the four detectors arranged according to a corner cube configuration cannot be conveniently attached and fixed to the surfaces of the corner cubes without specially designed mechanical structure and well trained alignment skill. On the other hand, the hollow structure requires the detectors positioned in the surfaces of the corner cube further prevent it from minimization of the size of the photopolarimeter.
In the meantime, the demand for mass production of the photopolarimeters is increased, particularly for application in ultra long haul optical transport systems. In the attempts to increase the spectral efficiency by putting similar number of channels (compared with 10 G systems) in one fiber with a date rate of 40 Gbps/channel, and to maintain a competitive price by both keeping the similar or longer distance of transmission without any regenerators, dispersions of multiplexed optical signals over longer distance must be compensated. Innovative technology and manufacturing process to reduce the cost of components and modules such as photopolarimeter for measuring the SOP of light projection to accurately compensate the dispersions are in increased demand. Particularly, the challenges to the implementation of 40-Gbit/sec systems require a broad array of specially developed components and modules. In addition to chromatic dispersion compensation and high-speed electronics, polarization mode dispersion compensation (PMDC) will play an important and essential role in the system implementation. A PMDC device is able to correct the optical signal distortion caused by the degenerate of the two eigen modes of polarization. A PMDC when combined with chromatic dispersion compensation can enable a high date rate of transmission over a long distance. For that reason, a PMDC is usually put in front of a receiver to carry out a channel-by-channel compensation. A typical setup is given in FIG. 1. The polarization controller consists of two quarter wave plates sandwiched with a half wave plate, which their fast axes are rotated to convert any input polarization state into the desired output polarization. A polarization analyzer (photopolarimeter) is an essential module to provide the three control parameters of the rotation angles to fully control the PMD. In order to be suitable for practical and economical system applications, the module of polarization analyzer should be compact, real time, broadband, low cost, and with minimal insertion loss and polarization dependent losses (PDL) for in-line application.
Therefore, a need still exists in the art of manufacturing and designing the polarization analyzer to provide a new configuration to enable the manufacture of a photopolarimeter that is compact, real time responsive, operational over a broadband range, low cost, and with minimal insertion loss and PDL thus suitable for in-line application.